1. Field of the Invention
The present invention generally relates to optoelectronic devices, and more particularly, relates to optoelectronic devices having a sandwich structure and methods for forming the optoelectronic devices.
2. Description of Related Art
Organic optoelectronic devices, such as organic solar cells (OSC), organic light emitting diodes (OLED), organic light sensors, and so on, become increasingly advantageous according to the degree produced in light-weight, small-thickness, large-area, flexible, and low-cost forms.
In order to augment the power conversion efficiency of organic optoelectronic devices, a buffer layer can be disposed between the organic layer and the transparent electrode. For example, a thin layer composed of calcium or lithium fluoride can be disposed between the aluminum electrode and the organic layer. A buffer layer including, for instance, poly(3,4-ethylenedioxythiophene), or PEDOT, can be disposed between the transparent electrode and the organic layer to increase the power conversion efficiency.
However, an aluminum electrode, or a buffer layer of calcium or lithium fluoride, is susceptible to being oxidized in the presence of air, causing the resistance of the device to increase. On the other hand, a buffer layer of PEDOT may over time result in corrosion of the transparent electrode, causing the device to be damaged.
In order to overcome the problems described above, efforts have been made to replace the aluminum electrode with a high work-function metal to be used as an anode, and with transition metal oxides, such as vanadium oxide or tungsten oxide, being formed between the anode and the organic layer for transporting or injecting holes effectively so as to increase the power conversion efficiency. In addition, another transition metal oxide, zinc oxide, which is not corrosive to the transparent electrode, can be formed between the transparent electrode (as cathode) and the organic layer to be used as an electron transporting or electron injecting layer in place of PEDOT.
The transition metal oxide layers described above are usually formed by using a vacuum evaporation process, which is costly and difficult for producing a large-area device. Some transition metal oxide layers can be formed by the sol-gel method. While it is possible to produce a large-area device using the sol-gel method, the sol-gel method includes a high temperature annealing treatment. Consequently, the processing temperature is usually higher than the glass transition temperature (Tg) of the organic material, which may result in damage to the organic layer.